Slag-making methods and materials

ABSTRACT

A lime product is intimately combined with a halogen compound and preferably one or more other fluxes to form a reactive flux addition which is used in steel making. The flux addition is obtained by burning a CaCO3 bearing material in the presence of auxiliary fluxes or by briquetting a mixture of lime and auxiliary fluxes, including a halogen compound.

United States Paten [1 1 Fedock et al.

[451 Nov. 13, 197-3 SLAG-MAKING METHODS AND MATERIALS [75] lnventors: Michael P. Fedock, Seven Hills;

Robert W. Limes, Cleveland; Robert 0. Russell, Rocky River, all of Ohio [73] Assignee: Republic Steel Corporation,

Cleveland, Ohio 22] Filed: Dec. 3, 1970 21 Appl. No.: 94,898

[52] U.S. Cl 75/60, 75/30, 75/54, 75/93, 75/94 [51] Int. Cl C21c 5/36, C22b 9/10 [58] Field of Search 75/54, 93, 94, 55, 75/30, 60

[56] References Cited UNlTED STATES PATENTS 3,251,680 5/1966 Goss et al. 75/51 X 2,110,067 3/1938 Heuer 75/49 X 2,079,848 5/1937 Francis 75/54 X 2,855,291 10/1958 Litton 75/54 3,311,465 3/1967 Ban et al 75/54 X 2,159,977 5/1939 Nicholas.... 75/54 X 2,111,893 3/1938 King 75/54 X 2,587,573 2/1952 Wynne.... 75/51 2,767,078 10/1956 Perrin 75/54 3,212,881 10/1965 Dunn et a1. 75/49 X 3,320,052 5/1967 Bowden 75/54 X 3,585,025 6/1971 Obst et al. 75/54 2,102,119 12/1937 Henkel 75/54 2,597,851 5/1952 Bowden 75/54 3,313,617 4/1967 Ban et a1 75/54 X Primary Examiner-Byland Bizot Assistant Examiner-Peter D. Rosenberg Att0rney-Robert P. Wright and Joseph W. Malleck [57] ABSTRACT A lime product is intimately combined with a halogen compound and preferably one or more other fluxes to form a reactive flux addition which is used in steel making. The flux addition is obtained by burning a CaCO bearing material in the presence of auxiliary fluxes or by briquetting a mixture of lime and auxiliary fluxes, including a halogen compound.

2 Claims, 4 Drawing Figures PAIENTEDRBV 1 3 ms 3.77 1. 999 SHEET 1 OF 4 INVENTOR5. MICHAEL Q FEDOCK P055127 W. L/MES BY EOBEET o. RUSSELL Q/m Woggm em flwna 1 SLAG-MAKING METHODSAND MATERIALS BACKGROUND OF THE INVENTION The present invention relates generally to the art of steel making, and more specifically to new methods and materials which improve both the rate and the efficiency of slag development in steel making processes such as the basic oxygen process.

In recent years the basic oxygen process has rapidly replaced open hearth steel making because of the speed with which iron can be refined. The heat times for the basic oxygen process are well under one hour as compared to 4 to 6 hours for similar sized heats in the open hearth. The speed of the process demands rapid slag formation, and this necessitates the use of a lime product which will go into solution in the slag quickly. Economical considerations require that a rapid solution rate of the lime be obtained without the necessity of adding large amounts of auxiliary fluxes such as fluorspar.

The dissolution of lime in the basic oxygen process is adversely effected by the formation of dicalcium silicate which results from an undesirable reaction between the lime and silicates produced at an early stage in the process. Dicalcium silicate is a refractory material having a high melting point about 3800F. Since the lime is customarily added to the furnace in pebble form, the dicalcium silicate may coat the pebbles so to inhibit their dissolution and consequently slow the rate of slag development.

It has been a conventional practice to charge an auxiliary flux into the furnace for the purpose of facilitating dissolution of the lime. Fluorspar is a material commonly used. When added in the conventional manner, the amount and the cost of auxiliary fluxes required to accelerate the slag formation rate to the degree necessary in the basic oxygen process are excessive.

It has also been conventional to oxidize the iron to form iron oxide which acts as a fluxing ingredient to aid lime dissolution. This expedient of forming iron oxide decreases the yield and can result'in slags which are extremely errosive to furnace linings made of magnesite or dolomite.

SUMMARY OF THE INVENTION The present invention provides methods and materials which are effective to decrease slag making time and which make it possible to reduce the amount of fluxes required to make a heat of steel. The invention is particularly concerned with the composition, manufacture and use of a new flux material which has an improved rate of slag solubility and is thus suited for use in modern methods of producing steel by oxygen blowing, as for example the basic oxygen process. As is well known, in the production of steel by oxygen blowing, scrap and molten pig iron are serially charged to the furnace and then the blowing period is initiated and carried out to turndown. Lime and preferably one or more auxiliary fluxes are also added to the furnace in sufficient quantity for the formation of slag during blowing.

The invention is based on the discovery that the slag solubility of burned lime which is intimately combined with other fluxing ingredients, including a halogen compound, is faster than that obtained by adding the lime and other fluxes separately to the slag. In addition to achieving a rapid slag solution rate of lime, it has been found that the amount of auxiliary fluxes can be reduced in comparison to the amount required by normal practice.

According to one practice of the invention, lime is intimately mixed with a halogen compound and one or more fluxes selected from the group consisting of iron oxide, alumina, phosphorus pentoxide and manganese oxide, and the mixture is compressed into briquettes suitable for addition to a steel making furnace. An advantage of this practiceis that the briquetted mixture may constitute the entire flux requirements for making a heat of steel by the basic oxygen process. When added to the furnace as one ingredient of the briquettes, the amount of the halogen compound, for example, fluorspar, can be reduced by as much as 30 per cent in comparison to normal practice.

According to another practice of the invention, limestone and/or dolomite is calcined in the presence of a halogen compound, as by sintering or in a rotary kiln. Other fluxes, namely, iron oxide, manganese oxide, phosphorus pentoxide and or alumina, also may be included in the mixture prior to calcination.

In addition to exhibiting an improved slag solution rate, the halogen containing lime product comprehended by the invention produces more basic slag with attendant metallurgical advantages and the further advantage of prolonging the life of furnace linings. Since the refractory linings of basic oxygen furnaces are made of dolomite or magnesite, the acid slags which are developed will attack the linings if the lime does not go into solution fast enough. By increasing the rate of lime solution into the slag, the rate of wear on the refractory linings is reduced.

Other advantages obtained by the invention include a simplification of the manner of introducing the flux into the furnace, and an increased resistance to hydration, whereby the lime product has a longer shelf-life than ordinary lime.

Still other advantages and a fuller understanding of the invention will be had by reference to the accompanying drawings and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing slag solubility results of different halogen reacted lime products in comparison to ordinary lime;

FIG. 2 is a graph showing slag solubility results of CaCl reacted lime products;

' FIG. 3 is a graph showing slag solubility results of CaF reacted lime products; and

FIG. 4 is a graph showing the slag solubility results of a CaF, reacted lime product in comparison to lime and CaF added separately'to the slag.

DESCRIPTION OF THE PREFERRED EMBODIMENTS cent by weight. When the new material is used as the total flux charge during slag making, the lime product may be present in the flux material in an amount of 70 percent or more by weight.

The burned lime and/or dololime is reacted or inti' mately combined in a manner to be described with a halogen compound which is believed to minimize the formation of dicalcium silicate coatings on the lime product when it is added to the molten metal bath. Suitable halogen compounds which have been used successfully to improve the slag solution rate of burned lime and/or dololime include halogen salts or halides, such as CaCl NaCl, KCl, and CaF Similarly improved results are expected with the use of compounds of iodine and bromine. Fluorspar has been found to be particularly effective in improving the slag solution rate of lime and is the preferred halogen additive.

The new flux material is prepared from a mixture which contains a halogen compound in an amount of from about 0.5 to 15 percent by weight. Larger amounts of halogen compound may be used, but are generally uneconomic. In addition to the halogen compound, the mixture preferably contains auxiliary fluxes including iron oxide, manganese oxide, alumina, and phosphorus pentoxide. When reacted or intimately mixed with lime, such auxiliary fluxes in addition to the halogen are effective to further inhibit the formation of dicalcium silicate and thereby inprove the slag solution rate. The iron oxide may be finely divided ferric oxide (Fe o recovered from fume abatement devices, such as electrostatic precipitators, and may be present in the mixture in an amount up to about 30 percent by weight. If used, the manganese oxide, phosphorus pentoxide and alumina may be present in a combined or total amount up to about 30 percent by weight.

A preferred procedure of this invention involves briquetting a mixture containing in amounts by weight at least 50 percent lime, from O to 30 percent dololime (CaOMgO), from 0.5 to 15% halogen compound, and from I to 30 percent of at least one flux selected from the group consisting of iron oxide, A1 MnO, and P 0 A preferred briquetting mixture contains in amounts by weight at least 70 percent and more preferably from 77 to 83% CaO, at least 14 percent and more preferably from 14 to 16% CaO-MgO, from 2 to 4 percent fluorspar, and from 4 to 5 percent iron oxide. The lime and dololime used in preparation of the mixture should have a particle size no larger than 541 inch and more preferably should be less than it; inch. In order to assure good distribution of the halogen compound and other fluxes throughout the crushed lime, the flux addition should be at least minus 20 mesh and more preferably minus 100 mesh.

The briquettes of the foregoing specifications may contain the entire flux addition necessary to produce a heat of steel. As generally discussed above, the preferred use of briquettes containing lime, a halogen compound such as fluorspar, and at least one additional flux such as iron oxide makes it possible to substantially reduce the amount of fluorspar per ton of steel in comparison to the amount required when the lime and fluorspar are separately charged into the furnace. Exemplary heats made using briquettes of the preferred composition have averaged 30% less fluorspar per ton of steel compared to heats made by charging lime and fluorspar separately.

According to another preferred procedure of this in vention, the new flux material is prepared by mixing a halogen compound and any auxiliary fluxes which are desired with a CaCO bearing material, namely, limestone and/or dolomite, and heating the mixture to burn and reduce the CaCO content to lime. The calcination temperature should be below the boiling point of the halogen additive in order to react and diffuse it in the stone and thereby obtain a halogenated product exhibiting maximum slag solubility. The stone-flux mixture may be calcined in a rotary kiln or other conventional calcining apparatus, Alternatively, the mixture may be sintered in a conventional manner, as on a travelling grate sinter strand, to affect calcination.

Preferred mixtures for use in the above-described calcining practice consist of at least 50 percent limestone and/or dolomite, and more preferably are at least 50 percent limestone. Specifically preferred mixtures consist in parts by weight of at least 50 percent limestone, from 0 to 30 percent dolomite, from 0.5 to 15 percent halogen compound, from O to 30 percent iron oxide, and from 0 to 30 percent of a flux selected from the class consisting of MnO, A1 0 and P 0 For sintering use, the above mixtures are combined with coke or other fuel. In order to obtain good sintering rates and proper diffusion of the halogen additive into the lime product, it has been found desirable to use a stone having a maximum size of inch and a minimum size of plus mesh. An optimum stone size is 30 percent minus 3 plus 6 mesh, 40 percent minus 6 plus 20 mesh, and 30 percent minus 20 plus 100 mesh.

The following are specific examples of the preferred embodiments and practices of the invention.

EXAMPLE 1 A series of tests were conducted to show the improved slag solubility rates of a lime product produced by calcining a mixture of limestone and halogen additive. The tests were conducted using a homogeneous limestone having a particle size of minus l inch and plus )6 inch. Four sample mixtures were prepared to include a 2 percent by weight addition of the additive, the selected halogen compounds being NaCl, CaCl and CaF A fifth comparison sample consisted solely of the limestone without a halide addition. All samples were burned at a temperature of 2,150F. for one hour, cooled and then crushed to a size of minus 5 plus 6 mesh.

The crushed samples were reacted with a lime deficient slag and the rates of solution into the slag were determined. This was done by mixing the burned lime samples with lime-deficient slag-forming mixture in a proportion of 3 parts lime grain to 1 part slag mixture. The slag-forming mixture was a synthetic composition simulating an early BOF slag without lime, and had an approximate composition of 56% SiO 3.8% A1 0 3.7% TiO 19% F 0 8.5% MgO, and 9% MnO. A series of crucibles containing each of the lime grain-slag mixtures were placed in a gas fired furnace at 2,700F. Crucibles containing each mixture were withdrawn from the furnace at two minute intervals from 4 to 10 minutes. The crucibles were then crushed and the free lime determined by the sugar method.

FIG. 1 is a plot of free lime against time in the furnace and indicates the rate of lime solubility into the slag. It will be seen that in all cases limestone calcined with a halide addition yields a lime product superior to lime calcined in the absence of the addition. It will also be seen from FIG. 1 that the best solubility is obtained with the Cal addition.

EXAMPLE 2 Sample mixtures were prepared consisting of the limestone used in Example 1 and CaCl in amounts by weight of 0.5, 1.0, 3.0, 6.0 and percent. The five sample mixtures and a comparison sample consisted solely of limestone were calcined at 1,800F. for one half-hour. Slag solubility rates of the burned lime products were then determined in the same manner as described in connection with Example 1. The results of the slag solubility testing are plotted in FIG. 2 which shows that all additions favorably increased slag solubility.

EXAMPLE 3 Another series of sample mixtures were prepared using the same limestone as in previous examples with additions of various amounts of CaF and the mixtures were calcined at 1,800F. for one half hour. Slag solubility test results conducted with these sample mixtures are plotted in FIG. 3 and show an improvement in solubility with 'all additions of fluorspar.

EXAMPLE 4 A 2% Cal reacted lime was prepared in the same manner as in Example 3. Slag solubility tests were run on samples of the reacted lime and on samples prepared by adding 2 percent by weight addition of CaF to a lime grain-slag mixture. The tests also include comparison lime grain-slag samples with no CaF addition. The results are plotted in FIG. 4. As shown, the addition of CaF to the slag improved lime solubility after 10 minutes in the furnace. In all instances, the lime solution of the Calpre-reacted lime was far superior to that obtained by adding lime and CaF separately to the slag.

EXAMPLE 5 Raw Material Screen Analysis BOF ppt. Dust Dolomite Fluorspar Aim Coke Breeze Chemical Analysis Tot.Fe F130, FeO Mn SiO 1.9 1.31 6.60 A1 0, .57 1.60 P CaO 6.1 MgO .90 CaF 72.37 CaCO; 56.07 8.13 MgCo: 40.88

.20 .80 C 1.18 Zn .50 L01 4650 4.10 83.45 H 0 2.00 14.60 No./ft 70.8 85.0 134.0 33.00

*Bin contamination The mix was prepared in a pug mill according to conventional procedures, including wetting to glistening". This mix was then sintered on a conventional travelling grate sinter strand to obtain the desired product for use in basic oxygen process slag making operations. The chemistry of the sinter product was as follows: 4.14% FeO, 16.82% Fe o 3.68% SiO 0.9% A1- 0 34.10% CaO, 24.4% MgO, 10.83% CaF 0.09% P, and 0.33% Mn.

Results of a series of tests using the sinter product to make slag in a basic oxygen furnace confirmed better lime solution in comparison to a product made in the identical manner from a mix which did not include the CaF addition. The per cent of lime solution in the slag was calculated by the following formula:

CaO 1.19 P 0 measured Sio m 100=percent lime solution charged Using this formula, the sinter product with the Cal addition had a lime solution of 79.8 percent in comparison to a 74.9 percent lime solution for the sinter product with no Cal addition.

EXAMPLE 6 The briquettes were used in a basic oxygen furnace to supply the total flux requirements for the production of heats of steel or 14,768 tons of ingots.

All heats were of good quality. Tests run on the first 32 heats indicated a yield of 88.67 percent compared to an average yield of 87.53 percent from 32 heats which were made by charging lime and auxiliary fluxes separately in the furnace according to conventional practice.

It was found that when the flux was added in the form of briquettes less flux was required per ton of steel than when the lime and auxiliary fluxes were charged separately. More particularly, the heats made with the briquettes averaged about 168 pounds of flux per ton of steel compared to an average of about 233 pounds of flux per ton of steel in conventional practice.

Other advantages resulting from the use of the briquettes included less slopping, fewer reblows and less slag volume. The heats shaped up faster and were smoother to blow. The material was uniform and deb. charging lime and auxiliary fluxes into the furnace pendable, and the slag was fluid on all heats. in sufficient quantity for the formation of slag dur- Many modifications and variations of the invention i l i will be apparent to those skilled in the art in the light c said me and auxiliary fl being added to the of the foregoing detailed disclosure. It will be understood therefore, that the invention can be practiced otherwise than as specifically described.

What is claimed is:

l. A method of producing steel by oxygen blowing furnace in the form of briquettes consisting essentially of the following ingredients in amounts by weight: over 50 percent lime, from 0.5 to 15 percent of at least one halogen compound, and from 1 to 30 percent of at least one material selected comprising: 10

a. the Serial Steps from the class consisting of iron oxide, MnO, Al O i. charging scrap into a steel making furnace, and 2 5 ii. charging hot met l i t th furnace, d 2. A method as claimed in claim 1 wherein said briiii. blowing by directing oxygen on or into the quettes contain at least 70 percent by weight lime.

charge, and 

2. A method as claimed in claim 1 wherein said briquettes contain at least 70 percent by weight lime. 